1. Field of the Invention
This invention relates in general to radio transceivers, and more particularly to a method and apparatus for improving loop stability and speed of a power control loop.
2. Description of Related Art
Cellular mobile radio provides the technology that enables everyone to communicate anywhere with anybody. This technology has created an entire industry in mobile telecommunications, which is rapidly growing and has become a backbone for business success and expanding economies.
Telecommunication devices are designed to move information from one place to another over channels, and a radio channel is an extraordinarily hostile medium on which to establish and maintain reliable communications. A channel is particularly noisy and unruly between mobile radios. If the number of channels available for all the uses of a radio system is less than the number of all possible users, then such a system is called a trunk radio system. Trunking is a process whereby users share a limited number of channels in some orderly manner.
Time division multiple access (TDMA) is one method for sharing channels. TDMA implies the use of digital voice compression techniques which allows multiple users to share a common channel on a schedule. Modern voice encoding greatly shortens the time it takes to transmit voice messages by removing most of the redundancy and silent periods in speech communications. Thus, multiple users can share the same channel during the intervening time slots. Accordingly, all users share the physical resource by having their own assigned, repeating time slot within a group of time slots called a frame. For this reason, a time slot assignment is often called a channel.
Within a single cell, mobile stations can be found at different distances from a base station. In a cellular communications system, mobile stations are dispersed at varying distances relative to a base station. Depending on the distance to the base station, the delayed time and the attenuation of an individual""s mobile signal is likely to be different from the delay and attenuation of any of the other mobile stations. However, TDMA techniques rely heavily on the proper timing of transmission bursts. Thus, a base station performs measurements on the time and delay of each mobile station. The base station then commands these mobiles to modify their transmission bursts. This feature is called timing advance.
To compensate for attenuation over different distances within the cell, the base station, at the same time it is making timing adjustments on mobiles, commands the mobiles to use different power levels in such a way that the power arriving at the base station""s receiver is approximately the same for each time slot. This power control is typically performed in steps of 2 dB. To perform this power measurement process, the base station gives the mobile station a list of base stations on which to perform power measurements. The mobile station performs continuous measurements on the quality and the power level of the serving cell, and of the power levels of the adjacent cells. The measurement results are put into a measurement report, which are sent back to the base station. The base station may also perform measurements on the quality and power of the link to the mobile station. If a base station discovers that a mobile station is not receiving its signal in sufficient power level for reliable downlink communication, the base station may also apply a power control on its own by transmitting power levels in each time slot.
In TDMA, transient response and loop speed are important in the design of the control loop. However, loop stability and loop speed are two contradictory design parameters in control loop theory. For example, the wider the loop bandwidth, the faster the loop speed, but the lesser the loop stability margin.
It can be seen then that there is a need for a power control loop method and apparatus which has reduced loop bandwidth variation so that loop stability and loop speed may be maximized.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for improving loop stability and speed of a power control loop.
The present invention solves the above-described problems by reducing adjacent channel interference in the ramp-up and ramp-down of a TDMA burst. The present invention is used when the transient response and loop speed are important in the design of control loop. The loop bandwidth variation resulting from linear conversion power detector and PIN diode RF attenuators in the loop is reduced by reducing the upper end of the loop bandwidth and increasing the lower end of the loop bandwidth. The upper limit of the loop bandwidth determines the loop stability and the lower limit of the loop bandwidth determines the minimum speed of the loop. Accordingly, the present invention not only improves the loop stability, but also the speed of the loop.
A system in accordance with the principles of the present invention includes a power control loop for reducing adjacent channel interference in a ramp-up and a ramp-down of a signal burst in a cellular communication system. The power control loop having a feedback loop controls the ramp-up and the ramp-down and has a predetermined bandwidth, wherein a reference signal is sampled and processed to set a gain of the feedback loop in the power control loop, the setting of the gain of the feedback loop using the reference signal increases a lower loop bandwidth to increase a loop speed and decreases an upper loop bandwidth to increase a loop stability for the power control loop.
Other embodiments of a system in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that a power control loop includes a first amplifier for receiving an input signal and generating a power output signal in response thereto, a coupler for sampling the power output signal, a detector/linearizer circuit, coupled to the coupler, for generating a voltage signal, a comparator, coupled to the detector/linearizer circuit, for receiving the voltage signal and comparing the voltage signal to a reference signal to generate an error signal, a variable gain amplifier, coupled to the comparator, having a selectable gain, the variable gain amplifier receiving the error signal to produce a controller input signal, a gain controller, coupled to the variable gain amplifier, for ramping the gain of the variable gain amplifier by processing the reference signal and a controller, coupled to the variable gain amplifier, the controller receiving the controller input signal and generating a control signal to control attenuation of the input signal to the first amplifier, wherein the ramping of the gain of the variable gain amplifier provides a constant loop bandwidth of the power control loop without regard to the power level of the sampled power output signal.
Another aspect of the present invention is that the ramping of the gain of the variable gain amplifier causes a decrease to an upper loop bandwidth of the power control loop and an increase to a lower loop bandwidth of the power control loop.
Another aspect of the present invention is that the ramping of the gain of the variable gain amplifier lowers bandwidth variation of the power control loop to decrease adjacent channel interference.
Another aspect of the present invention is that the ramping of the gain of the variable gain amplifier is synchronized with the reference voltage.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.